Woven papermaker fabrics are intended to be used in the sections of a paper making machine as a support for the paper web. They are endless woven fabrics or fabrics made endless by means of a seam which circulate in the paper making machine. In the first section of a paper making machine, forming fabrics are used, on the top surface of which a paper pulp is disposed at the start, and the latter is dewatered through the forming fabric such as to form a paper web, wherein the dewatering is supported in particular by suction boxes disposed on the bottom or machine side of the fabric.
A paper maker fabric must have a fine structure on the paper side in order to support and retain a high percentage of the deposited paper fibers. On the other hand, the machine side of such a fabric must be durable enough to withstand wear and give adequate life, strong enough to resist tensile forces and open enough to provide drainage. Meeting these criteria generally requires that at least two fabric layers are superimposed utilizing threads of different size and/or count per cm and differing weave patterns.
Generally known are three types of paper maker fabrics used for the forming section of a paper making machine. A first type is constructed of an upper and a lower layer out of transverse threads, the threads of both layers are superposed in pairs. The fabric layers are connected by longitudinal threads mostly interwoven in the upper layer and only a portion of them additionally interwoven in the lower layer, the threads of which are preferably of a larger thickness than the longitudinal threads. Such kind of forming fabrics are commonly called double-layer fabrics.
The second type of fabrics is created from two distinct fabrics, one having the qualities desired for the paper side and the other the qualities desired for the machine side. The two fabrics are stitched or bound together by additional or independent binder yarns, which do not belong to the recurring woven fabric repeats formed by the longitudinal and transverse threads of both layers. This type of papermaker fabric is commonly called a triple-layer fabric.
The third type of papermaker fabric is based on the second type, i.e. has two distinct fabric layers comprising longitudinal and transverse threads interwoven with one another respectively. Unlike the triple-layer fabrics, this type of fabric has no additional or independent binder threads. The connection of the two layers is made of their own threads, i.e. by so-called “structural”, “intrinsic” or “fabric-born” threads. These are threads which are an integral part of the weave pattern of the fabric or its woven fabric repeats. Composite fabrics of these types are commonly called SSB (sheet support binder) fabrics.
Differences between the foregoing three types of fabrics are also described in U.S. Pat. No. 5,152,326, columns 1 and 2, U.S. Pat. No. 5,052,448, columns 1 through 3 and U.S. Pat. No. 4,554,953, columns 1 through 3.
Paper maker fabrics of the SSB composite type are known, wherein the connection between the two layers is achieved by a number of intrinsic binder thread pairs, the binding of the one binder thread in one layer is continued by the other binder thread of the binder thread pair, if the one binder thread changes from one layer to the other layer in a symmetrical manner (U.S. Pat. No. 5,152,326; US 2008/0035230 A1; EP 1 605 095 A1 and EP 1 365 066 A1; US 2010/0258259 A1; U.S. Pat. No. 5,826,627; WO 2005/017254 A1). Also known are embodiments of composite papermaker fabrics, wherein the two layers are connected by intrinsic binder threads belonging to the first woven fabric layer (U.S. Pat. No. 4,564,051; U.S. Pat. No. 5,052,448; U.S. Pat. No. 4,554,953; EP 2 314 762 A1; JP 49010281 B; JP 62-78294; DE 298 07 274 U1 and EP 1 365 066 A1; EP 1 506 339 A1; WO 2006/020414 A1; EP 1 936 024 A1; DE 10 2004 016 640 B3). In the latter document, an embodiment is also disclosed, wherein the two layers are connected by binder threads of the second woven fabric layer forming the machine side layer.
With respect to SSB composite type paper maker fabrics as mentioned above, it is commonly known that the binder threads bind with only one second thread of the second woven fabric at a binding point. In order to obtain a strong connection between the two woven fabric layers, it is necessary to form a large number of the first threads in one direction as binder threads. The disadvantage is that the machine side of the paper maker fabric is uneven due to the large number of binding points where the binder threads changes to the second woven fabric layer and hence, create recesses or dimples. However, embodiments of SSB paper maker fabrics are known, wherein the binder threads bind with two or more adjacent second threads at one binding point (US 2010/0258259; EP 2 314 762 A1, column 3, lines 10 ff.; U.S. Pat. No. 5,437,315 A; U.S. Pat. No. 4,554,953 A) As indicated in EP 2 314 762 A1, these embodiments have the disadvantage that the first binder threads then have less presence in the first woven thread layer, i.e. larger recesses are produced at any binding point resulting in a more uneven surface on the paper side, which causes a worse paper fiber retention and a tendency of producing markings in the paper sheet.